1. Field of the Invention
The present invention relates to a system for controlling the depth of a seismic deflector under tow through the water.
2. Background of the Related Art
The ability to conduct accurate seismic surveys may help improve the discovery rates and even the production of subsurface accumulations, such as hydrocarbons. Seismic surveying is a method of stimulating a geological subsurface formation with, e.g., electrical, magnetic, and/or acoustic signals to acquire seismic data about the formation. From this data, one can predict whether the formation contains hydrocarbon deposits and, if so, where those hydrocarbon deposits are located.
One type of seismic survey is generally referred to as a “marine” survey, because it is typically conducted at sea, although this is not necessarily always the case. During marine seismic surveys, seismic cable systems are deployed in the water behind a towing vessel.
Deflector devices, also known as deflector systems (collectively “deflector(s)” herein), are used between a towing vessel and a streamer located in water, in order to pull the streamer out to one side of the vessel. Control of the deflector allows the streamer to be positioned at a desired lateral offset from the course followed by the vessel. Seismic surveys are generally carried out with a number of streamers towed in substantially parallel paths behind a vessel.
For example, FIG. 1 is an aerial view of a typical towed streamer array using a door type deflector. The system 10 includes a vessel 12 for pulling lead-ins 14 and streamers 15 through the water. A door deflector 16 is coupled by cables or “ropes” 18 to the streamers 15 or lead-ins 14. As the deflector 16 is towed through the water in the direction of tow indicated by the arrow 20, the force of the water against the surfaces of the deflector 16 allows the deflector to pull the streamers 15 out laterally to the side of the vessel 12. This allows the streamers 15 to be appropriately spaced over a larger survey area. Typically, the streamer array is symmetrical about the central axis 22 that extends directly behind the vessel.
FIG. 2 is a perspective view of a door deflector as shown in FIG. 1. The door deflector 16 is a traditional seismic deflector, also referred to as a vane, bi-vane, Baro-door, Baro vane, or paravane. The door deflector 16 comprises of a number of parallel vertical wings 24 mounted along side each other in a frame 26 that typically forms a rectangle. The door deflector 16 is normally towed with up to six bridle chains 28, including one chain 28 from each corner of the rectangular door and often two extra chains 28 in the middle. The deflector 16 is completely submerged and positioned generally vertically in the water by suspending the deflector by a chain 30 coupled a surface float (not shown).
FIG. 3 is an aerial view of towed streamers using a wing type deflector. The system 40 includes a vessel 12 for pulling lead-ins 14 and streamers 15 through the water. A wing deflector 42 is coupled between the lead-ins 14 and the streamers 14 and towed through the water in the direction of tow indicated by the arrow 20. The force of the water against the deflector 42 allows the deflector to pull the streamers 14 out laterally to the side of the vessel 12 into an appropriate spacing for a survey. Typically, the streamer array is symmetrical about the central axis 22 that extends directly behind the vessel.
FIG. 4 is a perspective view of a wing deflector as shown in FIG. 3. In use, the wing deflector 42 has a wing-shaped body 44 suspended by a chain or a rope 30 beneath a float (not shown) so as to be completely submerged and positioned generally vertically in the water. As the deflector device is pulled through the water, the wing-shaped body produces a sideways force, or “lift”, which moves the deflector laterally relative to the direction of tow. It is useful to define an “angle of attack” when discussing such lift, this angle being defined by the arc between the plane in which the trailing surface of the deflector body lies and the direction of tow through the water. The angle of attack will lie generally in a horizontal plane, although not necessarily so. Thus, in FIG. 4, the angle of attack is indicated as angle f between trailing deflector body surface 44a and direction of tow 20.
An exemplary wing deflector is described in detail in U.S. Pat. No. 5,357,892, which patent is incorporated by reference herein, and comprises a wing-shaped deflector body having a remotely-operable pivotal lever or “boom” which extends rearwardly from a point near the middle of the trailing edge of the wing-shaped body. The lift produced by the deflector can be varied by adjusting the angle of the boom from the vessel, thus permitting the lateral offset of the tow from the course of the vessel to be varied in use. The deflector device of U.S. Pat. No. 5,357,892 has been successfully commercialized by the Applicant as its MONOWING™ deflector device. In use, rolling stability of the device is provided by the connection to the float, while stability of the device about a vertical axis is provided by the drag produced by the tow.
A different version of the MONOWING exists where the angle of attack is controlled by other means than regulating the angle of the boom as described above and which relates to U.S. Pat. No. 5,357,892. In this system, a relatively long boom is rigidly fixed to the suction side of the wing and pointing rearwardly from the wing. In the rear end of this boom are mounted so called boom-wings that are adjustable in angle of attack and hence lift. By means of adjusting the lift of the boom-wing, new equilibrium positions in the so called yaw angle (rotation about the vertical axis) are found and the lift of the main wing is modified.
The MONOWING deflector devices in current use are very large, typically 7.5 m high by 2.5 m wide, and weigh several tons. They are usually suspended around 2 m to 8 m below the float by means such as a fiber rope, and are also provided with a safety chain intended to prevent separation of the float and wing-shaped body in the event that the rope breaks. In rough weather, the upper part of the wing-shaped body may rise up out of the water, allowing the rope connecting the wing-shaped body and the float to go slack. If the wing-shaped body then drops abruptly, the rope, and possibly even the safety chain, may break, and/or their attachment points on the wing-shaped body may be badly damaged.
The depth at which the current deflector devices operate is effectively determined by the length of the rope connecting the deflector to the float. As a result of this, the operating depth of the deflector device cannot readily be varied while the device is deployed in the water. And since the normal operating depth of the current deflector device is typically a few meters, in the event of the onset of bad weather during a survey, the device and all the streamers and other equipment directly or indirectly attached to it have to be recovered onto the towing vessel, and then re-deployed when the bad weather has passed, both of which operations are very time consuming.
Therefore, there is a need for a deflector that can be controlled to a given depth. It would be desirable if the depth were controllable on a continuous basis. It would be further desirable if the deflector was not directly affected by wave actions. It would be even more desirable if depth control could be used with existing deflector designs, including both door and wing deflectors.